Computer and other electronic assemblies typically include a plurality of printed circuit (PC) boards that support electronic components. Commonly, PC boards are secured within a steel or plastic chassis for an electronic assembly by means of screws extending through holes in the PC board and threadedly engaged to a portion of the chassis. In addition to the mounting screws, the assignee of the present invention also has used an alternative PC board mounting technique by which vertically-extending plastic hooks are molded into the base of the computer chassis. The hooks are received through slots formed in a PC board as the PC board, lying in a horizontal plane, is lowered onto the base of the chassis. After the PC board is seated on the chassis, with hooks extending through slots, the PC board is displaced horizontally to engage the hooks on the upper surface of the PC board at one end of the slots, thereby preventing vertical motion of the PC board. Thereafter, a small plastic catch mechanism is snapped into engagement with an edge of the PC board to prevent horizontal motion.
Because of the sensitivity of electrical components, and the risk that electrical circuits may be shorted if not properly secured within the product, it is necessary to ground the circuit board and to provide a ground for the electrical components on the circuit board. In many instances, both of these functions are provided by the fasteners (screws and/or hooks) that extend through the circuit board to the chassis of the product. Thus, for example, in a personal computer chassis, the motherboard containing many of the computer's electrical components is fastened to the chassis by a plurality of mounting screws and/or hooks. Because the chassis usually is constructed of a conductive material, or contains a conductive layer, a path to ground is provided through the mounting fasteners. The grounding of the circuit board is necessary to provide shielding against electromagnetic interference ("EMI") from outside sources, which might otherwise disturb the operation of the components on the circuit board.
As one skilled in the art will understand, a large variety of electrical components may be mounted on the circuit board, including integrated circuits and other discrete components. Typically, the electrical components are mounted on the circuit board in one of two ways. The first technique for mounting electrical components on a circuit board is to provide electrical leads on the electrical components that extend through the circuit board. The tips of these leads then are bent and/or soldered to an outer surface of the circuit board. Where space is limited, solder usually is preferred as a method to both secure the electrical component to the circuit board, and to provide a high quality electrical connection between the leads and the circuit paths on or in the circuit board. In order to perform this soldering process, usually a wave soldering machine is used to provide the requisite soldering of connections on one or both sides of the circuit board. Typically, the circuit board is placed in the wave soldering machine so that solder is applied to all conductive surfaces on the bottom of the circuit board.
In addition to providing solder to the leads of the electrical components, solder also is applied to conductive pad surfaces on the solder-side of the circuit board during the wave soldering process. To the extent that conductive surfaces on the solder-side of the board are not to be soldered, a solder mask must be provided as the bottom layer of the circuit board. The solder mask resists the application of solder. Thus, in accordance with normal techniques, the solder-side surface of the circuit board is designed with conductive pads, such as copper, where solder is to be deposited during the wave soldering process. Some of the larger conductive pads that are provided on the bottom of the circuit boards are the grounding pads, where grounding screws and/or hooks are to be attached or mounted.
Printed circuit boards typically are designed in layers, with various circuit paths, and in some instances, circuit elements, in each layer of the board. Thus, a typical circuit board has a number of different layers, with the upper and lower layers including circuit paths for connecting to components that are mounted to the top or bottom of the circuit board. In prior art circuit boards, typically a plurality of "plated through holes" are provided for receiving the grounding screws. As that term is conventionally used, plated through holes are holes drilled through the circuit board, which include a conductive material applied to the board at the periphery of the mounting hole so that each layer of the circuit board can be electrically connected to the grounding screw. In addition, a solid conductive (copper) pad typically is provided on external layers of the board adjacent the aperture for the screw. The upper pad functions to connect to the head of the screw, while the lower pad functions to connect to the body or chassis of the product in which the board is mounted. Solder tends to wick through the plated through holes, and to accumulate in large mounts on the relatively large pads. Because solder is not applied evenly in the wave solder process (based upon various considerations such as surface tension, etc.), after soldering the solder deposited on the conductive pads and plated through holes tends to be uneven, and in fact, relatively large bumps or clusters may appear at one portion of the pad while other portions have relatively large valleys. This problem is greatly exaggerated on larger pad areas, such as the grounding pads.
Because of these abnormalities in solder heights at the grounding pads and plated through holes, the installation of the boards may prove difficult, if not impossible. For example, in assignee's personal computers, hooks are used to secure the motherboard to the chassis. The hook therefore functions as a clamp by sandwiching the motherboard in place. See U.S. application Ser. No. 08/179,806, filed Jan. 11, 1994, the teachings of which are incorporated by reference as if fully set forth herein. The hook has a certain clearance to accommodate the circuit board. If the conductive pads at the site of the hook are too high, the circuit board will not fit within the hook structure. If, conversely, the height of the solder is too low, the conductive pad will not make a good contact with the hook and/or the metal chassis of the product in which the board is mounted, causing potential problems with electromagnetic interference. Moreover, if the height of the solder varies from one pad to another, the board will not lie flat within the chassis, but instead will be required to flex and bulge to fit properly in the chassis, causing stress on the circuit board. To alleviate these problems it typically is necessary to manually touch-up the larger solder pads in order to remove excess solder, or to add solder to provide a uniform solder height. This manual rework of the PC board adds significantly to the cost of manufacturing the circuit boards. Consequently, it would be advantageous to develop a technique for uniformly depositing solder on a circuit board during a wave soldering process to provide a solder joint with sufficient quantities of solder to provide adequate EMI protection, without providing too much solder to create problems with installation.
A second common technique for attaching electrical components to circuit boards is to surface mount the components by placing the component on conductive solder pads on the surface of the board. The solder then is heated in an appropriate chamber to reflow the solder on each pad to obtain a soldered connection with an associated lead or pad on the component. During the assembly of the circuit board, solder paste typically is deposited on the board by an automated (solder paste screening) machine, so that the solder paste is deposited in a relatively uniform manner to all of the conductive surfaces on the board. In addition to providing conductive paths for connecting to the leads of the surface mount components, it is common to also deposit solder paste on larger conductive pads where grounding is necessary, for example, where grounding screws and other attachments are to be located.
In the situation where the conductive pads are relatively large, the solder paste deposited on these pads tends to flow unevenly over the entire pad boundary when the reflow process occurs, leaving minimal or no solder height to electrically connect to the components provided on the chassis for grounding. There is a need to control and maintain the flow of the solder during the reflow process, to achieve enough solder raised above the surface in a desired area to provide a high quality electrical connection.
In many applications, such as computer motherboards, both types of components (surface mounts and through-hole) are used on the same circuit board. Typically, the surface mount components are soldered first on one side of the board, and then the board is placed in a wave solder machine to wave solder the other side of the board to connect the through hole devices to circuitry on the circuit board. In this situation, the problems with surface mount soldering and wave soldering are both present.
It obviously would be advantageous to develop a method for constructing a circuit board which would provide a uniform solder height for the larger solder areas to facilitate ease of installation and to insure adequate grounding for EMI protection regardless of the type of soldering that is used or the type of circuit board that is implemented.